Exam 2 Summer 2005 Solutions - For all problems, use...

Physics 221 2005 Su Exam 2 Page 1 of 7 For all problems, use g=10m/s 2 The correct answers are indicated in blue italics . 16. (1 point) In which of the following solid objects of uniform density does the center of mass NOT lie within the mass of the body? A. A cube B. A doughnut C. A sphere D. A cylinder E. None of the above A simple examination of the symmetry should be enough to give the answer to this one. 17. (1 point) An 80 kg man standing on a frictionless surface kicks forward a 0.04 kg stone lying at his feet so that it acquires a velocity of +4.0 m/s. What velocity does the man acquire as a result? A. +0.002 m/s B. +0.09 m/s C. -0.002 m/s D. -0.09 m/s E. None of the above. Momentum is conserved, and the initial momentum is zero, so 0f man man stone stone p m v m v = = + . Solving this for v man and plugging in the numbers gives the answer. 18. (1 point) Two masses of 1 kg are placed at x=4, y=5 and x=-4, y=5 and a 3 kg mass is placed at the origin, as shown in the figure below. Where on the Y-axis will the center of mass lie? Y X 3 kg (0,0) 1 kg (-4,5) 1 kg (4,5) A. y =1 B. y=2

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Physics 221 2005 Su Exam 2 Page 2 of 7 C. y =3 D. y =4 E. y =5 First take the CM of the two 1 kg masses. It appears like a mass of 2 kg at the point (0,+5). Then, combining this with the 3 kg mass, we find that the overall CM is at y=2 , x=0 . 19. (1 point) Two masses collide as shown in the figure below. Their masses and velocities before and after the collision are as shown. Which of the following choices describes the collision? Before: After: 0.5 kg 0.5 kg 2.0 kg 2.0 kg 10 m/s-3 m/s-10.8 m/s 2.2 m/s A. Inelastic B. Explosive C. Elastic D. Not enough information The quick way to get the answer is to observe that the relative velocities are unchanged, which is one of the characteristics of an elastic collision. Alternatively, you can calculate the kinetic energies before and after the collision and see that they are the same. 20. (1 point) What is the velocity of the center of mass of the particles in question 19? A. -0.4 m/s

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